As TTT keeps on telling us all, NJ, the population of Scotland is only 7% of the total UK's, so presumably on that basis theirs will be only 7% of the national debt. In addition, I trust you will take into account all the current jointly-owned resources that would inevitably - because of geography - belong to the 'rump' of what remains of the UK in the event of Scottish independence.
For example, the current Palace of Westminster was created as the British, not the English, parliament; clearly, however, Scotland will be unable to 'uproot' its 7% and drag it away to Edinburgh! Similarly, the so-called Bank of England is actually the Bank of UK and, despite what Gordon Brown may or may not have done to the gold therein, there is still some left, I'm sure! Apart from these two, there is a multitude of similar joint possessions. So, obviously the Rump, as I choose to call it, will have to "buy the Jocks out", as it were, from all of those, won't it? That should offset quite a chunk of what you choose to call "their share of the present national debt".